A communication method as a successor of the W-CDMA scheme and the HSDPA scheme, namely, the LTE (Long Term Evolution) scheme has been considered by the W-CDMA standardization organization 3GPP, and the specification setting work is underway.
As a radio access scheme of the LTE scheme, use of the OFDMA in the downlink and the SC-FDMA (Single-Carrier Frequency Division Multiple Access) in the uplink is under consideration.
The OFDMA is a scheme for dividing a frequency band into plural narrow frequency bands (sub-carriers) and transmitting data loaded on the respective divided frequency bands. In this scheme, high-speed transmission is achieved and frequency utilization efficiency is improved by arranging sub-carriers densely on the frequency bands in such a manner that the sub-carriers partially overlap but do not interfere with each other.
The SC-FDMA is a transmission scheme which reduces interference between terminals by dividing a frequency band and transmitting data by using frequency bands different among plural terminals. The SC-FDMA has a feature of less fluctuation in the transmission power, which achieves low power consumption and wide coverage of terminals.
The LTE scheme is a system in which plural mobile stations perform communication by sharing one or more physical channels in both uplink and downlink.
A channel shared by plural mobile stations is generally called a shared channel, which is, in the LTE scheme, a “Physical Uplink Shared Channel (PUSCH)” in the uplink and a “Physical Downlink Shared Channel (PDSCH)” in the downlink.
Also, such a shared channel is, as a transport channel, an “Uplink Shared Channel (UL-SCH)” in the uplink and a “Downlink Shared Channel (DL-SCH)” in the downlink.
In such a communication system using shared channels as described above, it is necessary to select mobile station UE to which the shared channel is to be allocated, and to transmit, to the selected mobile station UE, a signal indicating the allocation of the shared channel, for each sub-frame (1 ms, in the LTE scheme).
In the LTE scheme, a control channel used for the signaling is called the “Physical Downlink Control Channel (PDCCH)” or “Downlink L1/L2 Control Channel (DL L1/L2 Control Channel)”.
Meanwhile, the processing for each sub-frame of selecting mobile station UE to which the shared channel is to be allocated is generally called the “scheduling”. In this case, the processing may also be called the “dynamic scheduling”, because the mobile station UE to which the shared channel is allocated is dynamically selected for each sub-frame. Furthermore, “allocating the shared channel” described above may be alternatively expressed as “allocating a radio resource for the shared channel”.
Information of the physical downlink control channel includes, for example, the “downlink scheduling information”, “the uplink scheduling grant”, and the like.
The downlink scheduling information includes, for example, downlink resource block allocation information, ID of UE, the number of streams, information on the precoding vector, data size, modulation scheme, information on the HARQ (hybrid automatic repeat request), and the like, on the downlink shared channel.
Meanwhile, the “uplink scheduling grant” includes, for example, uplink resource block allocation information, ID of UE, data size, modulation scheme, uplink transmission power information, information on demodulation reference signal in the uplink MIMO, and the like, on the uplink shared channel.
Note that, the “downlink scheduling information” and the “uplink scheduling grant” described above may be collectively referred to as “downlink control information (DCI)”.
In the LTE scheme, the HARQ is applied to a communication using the shared channel described above. For example, on the downlink, the mobile station UE decodes the downlink shared channel, and transmits, to the radio base station eNB, acknowledgement information (ACK/NACK) based on the decoding result (CRC check result) by using the physical uplink control channel (PUCCH).
Then, the radio base station eNB performs retransmission control according to a content of the acknowledgement information, which is expressed either with a positive response (ACK) indicating that the transmitted signal is received properly or a negative response (NACK) indicating that the transmitted signal is not received properly.
FIG. 8 shows the downlink dynamic scheduling and the HARQ processing in a mobile communication system of the LTE scheme described above.
In the sub-frame #3, the radio base station eNB transmits, to the mobile station UE, downlink scheduling information via a PDCCH and downlink data via a PDSCH.
Then, the mobile station UE receives downlink data via the PDSCH, on the basis of the downlink scheduling information received via the PDCCH.
In the sub-frame #7, the mobile station UE transmits the acknowledgement information for the downlink data using a PUCCH, and the radio base station eNB receives the acknowledgement information (ACK/NACK) mapped to the PUCCH.
The radio resource of the PDSCH described above is dynamically allocated as being notified by the PDCCH. Also, the radio resource of the PUCCH described above is associated with the radio resource number of the PDCCH described above. Dynamic allocation of such radio resource of the PDCCH also results in dynamic allocation of such radio resource of the PUCCH.
That is, in the normal downlink scheduling (dynamic scheduling) of a mobile communication system of the LTE scheme, the radio base station eNB is configured to dynamically allocate a downlink radio resource and an uplink radio resource to the mobile station UE through the PDCCH, the downlink radio resource being for transmitting downlink data to the mobile station UE, the uplink radio resource being for transmitting acknowledgement information for the downlink data.
Furthermore, in the dynamic scheduling described above, a time difference between a sub-frame from which a PUCCH signal is transmitted and a sub-frame from which a PDCCH signal and a PDSCH signal are transmitted is fixed.
On the other hand, in the “persistent scheduling” that is under consideration to achieve the VoIP and the like, the radio base station eNB is configured to start a persistent allocation of the downlink radio resource (PDSCH) to a mobile station in the first cycles, at a sub-frame (first allocation starting time) in which the downlink scheduling information is transmitted to the mobile station UE through the PDCCH, and to persistently allocate the uplink radio resource (PUCCH) to the mobile station UE through the upper layer (Radio Resource Control: RRC) signaling.
In the “persistent scheduling”, the downlink scheduling information is transmitted via the PDCCH in a first transmission only, and the downlink scheduling information is not transmitted via the PDCCH in the subsequent transmissions. For this reason, the method for associating a radio resource of the PUCCH with a radio resource number of the PDCCH cannot be applied unlike the “dynamic scheduling” described above.
Accordingly, the uplink radio resource (PUCCH) in the “persistent scheduling” is persistently allocated to the mobile station UE by using the upper layer (RRC) signaling.
Here, the uplink radio resource represents, for example, a code resource in the code multiplexing or a frequency resource in the frequency multiplexing.
The frequency resource may be designated by a resource block number of a resource block (set of sub-carriers) from which the PUCCH is transmitted.
Furthermore, when a plurality of acknowledgement information are multiplexed within one resource block, the uplink radio resource may be designated by a predetermined identification number. For example, an identification number may be used to designate an amount of cyclic shift in the multiplexing of CAZAC sequence cyclic shift or an orthogonal cover code in the block spreading.
Furthermore, generally in the HARQ, a time difference between a time point where the above-described downlink radio resource (PDSCH) is allocated and a time point where the above-described uplink radio resource (PUCCH) is allocated is fixed. Accordingly, a transmission timing of the PUCCH is uniquely determined by designating a first allocation starting time through the PDCCH.
In the example shown in FIG. 9, the downlink radio resource (PDSCH) described above are persistently allocated in cycles of 20 ms, and the uplink radio resource (PUCCH) described above is persistently allocated for each downlink radio resource (PDSCH).
Specifically, the downlink radio resource (PDSCH) is persistently allocated in the sub-frames #3, #23, . . . while the acknowledgement information is transmitted in the sub-frames #7, #27, . . . .
Note that, in the sub-frame #3 of the example shown in FIG. 9, a first allocation starting time is designated by the PDCCH.